The present invention relates to a security film for shatterproofing windows against impact or explosion without normally detracting from their optical clarity, and, more particularly, to a security film for ready application to the inner face of a window as a safeguard against fragmentation of glass notwithstanding its splintering or cracking when damaged from outside. Prior optically clear security films, when adhered to window faces, have tended to rip in the event of impact or explosion to the accompanyment of flying glass, which is the primary danger that safety film is intended to prevent.
In accordance with the present invention, it has been found that 21/2 to 6 mils (62.5 to 150 microns) thick laminated assemblage (not including any release stratum) of alternate polyethylene terephthalate strata and inner and outer bonding strata, provides for radically improved shatterproofing when applied to the inner face of an architectural window, if (1) each of at least two of the polyethylene terephthalate strata is at least 1 mil thick, (2) each of the inner bonding strata between the polyester terephthate strata is composed of from 3.2 to 9.6 grams per square meter of a copolyester bonding agent from 0.1 to 0.3 mil (2.5 to 7.5 microns) thick, and (3) the outer bonding stratum for adhering the assemblage to a glass window is composed of from 9.6 to 38.4 grams per square meter of a pressure-sensitive acrylate polyene from 0.4 to 1.5 mil (10 to 37.5 microns) thick. All of these strata are optically clear and water vapor permeable and, optionally one or more of the polyethylene terephthalate strata is coated with a vacuum vapor depositied semi-transparent aluminum stratum. Based on the overall thickness of the assemblage, the copolyester stratum ranges from 6 to 18 percent and the pressure-sensitive stratum ranges from 13 to 26 percent. While the basis for the shatterproofing efficacy of this assemblage is not understood with certainty, it is postulated that this assemblage possesses a critical balance of physical properties including: sufficient overall flexibility to permit high incremental pressure when squeegeed onto a glass window; sufficient tensile modulus, impact strength and tear strength imparted by the oriented polyester strata to preclude rupture while distributing shock; particular adhesion, cohesion, compatibility and resilience to withstand shock without failure; sufficient modulus of rigidity discontinuities between the polyester terephthalate strata and the inner bonding strata to damp vibratory energy transfer; and particular adhesion, cohesion, compatibility and resilience imparted by the pressure-sensitive bonding stratum to catch and retain glass chards that are produced by deformation of the security film-window combination at the moment of initial impact.